SIL: Fill in unimplemented logic in verifyDynamicPackIndexStructuralEquality()

However, it should probably do something more elaborate where
it recovers the conformance from the pack conformance instead.

Author: slavapestov

lib/SIL/Verifier/SILVerifier.cpp

@@ -5719,10 +5719,9 @@ class SILVerifier : public SILVerifierBase<SILVerifier> {
       auto substConformances = [&](CanType dependentType,
                                    Type conformingType,
                                    ProtocolDecl *protocol) -> ProtocolConformanceRef {
-        // FIXME: substitute back to the pack context to get the
-        // corresponding conformance pack, then project out the
-        // appropriate lane.
-        llvm_unreachable("unimplemented");
+        // FIXME: This violates the spirit of this verifier check.
+        return protocol->getParentModule()
+            ->lookupConformance(conformingType, protocol);
       };
 
       // If the pack components and expected element types are SIL types,